1. Field of the Invention
The present invention is related to perpendicular magnetic recording systems, and in particular, to reduction of spike noise associated with domain walls in the soft magnetic underlayer of the media.
2. Background Information
Recently, the trend in magnetic recording apparatus is to employ perpendicular magnetic recording. In perpendicular magnetic recording, the magnetic domains of the bits are aligned perpendicularly to the surface of the disk platter. This allows bits to be placed closed together on the platter, thus increasing storage density over that which has been achieved in conventional longitudinal recording. Further, longitudinal magnetic recording is approaching an upper limit of bit density due to the super paramagnetic effect. The super paramagnetic effect relates to the energy required to change the direction of the magnetic moment of fine particles. The magnetic moment of the fine particles is affected by ambient thermal energy. Thus, at a certain point, the rate at which particles randomly reverse direction becomes significant. This is particularly relevant in hard disk technology because if the particles randomly reverse direction due to the super paramagnetic effect, then data is lost over time. Thus, the super paramagnetic effect limits the minimum size of particles that can be used without a significant loss of data over time. Perpendicular recording, however, changes the geometry of the disk and thus allows a greater bit density with lesser disadvantages due to the super paramagnetic effect. Accordingly, perpendicular magnetic recording has become a more favored recording technique in hard disk technology.
Perpendicular magnetic recording utilizes a soft magnetic underlayer (also referred to as the “SUL”), which serves as a conduit for the magnetic recording flux. The flux emanates from a monopole writing element, and is directed through the recording layer and returns through the soft underlayer and back to the writing element. Thus, it is desirable that the soft magnetic underlayer exhibit low coercivity, high saturation magnetization and moderate but constant permeability in the range of the write fields.
However, demagnetizing fields at the outer and inner edges of a magnetic disk give rise to the formation of complex closure domain structures (“domain walls”) which can interfere with the storage medium flux and result in a phenomenon known as “spike noise.” Specifically, domain walls form between adjacent regions in the soft underlayer that are magnetized radially outwardly and radially inwardly. In between the two regions, the magnetization must cross a domain wall, that is, change direction, which causes the magnetic flux to rotate outwardly in a perpendicular orientation and then back down to the plane of the disk. While the flux is pointed generally perpendicular to the disk, the magnetic flux creates a spike, (commonly known as “spike noise”). The spike noise can reach the read head and cause errors. The spike can be approximately 1 micron wide, and can thus affect about 40 bits of data, possibly creating read errors in those 40 bits of data.
Some techniques have been suggested for eliminating spike noise by layering the soft underlayer for better reduction of demagnetizing fields. Such methods may involve using a hard bias layer as one of the layers within the soft underlayer. However, this requires special manufacturing of the disk, thereby increasing the complexity and cost of manufacture. In other techniques additional field-setting steps are taken during disk manufacture to align the soft underlayer moments in a radial direction. These additional steps also involve additional complexities and costs.
There remains a need, therefore, for a cost effective method and apparatus for eliminating domain walls in a soft underlayer, thereby reducing the effects of spike noise in perpendicular magnetic recording media.